1. Field of the Invention
The present invention relates to a toner for use in developers which develop electrostatic latent images formed by electrophotography, electrostatic recording and electrostatic printing. More particularly, the present invention relates to a toner for use in developers for mono-color or full color image forming apparatus using a direct or indirect electrophotographic image forming method, such as copiers, laser printers and plain paper facsimiles. In addition, the present invention also relates to a method for preparing the toner, and an image forming method and an image forming apparatus (such as a process cartridge) using the toner.
2. Discussion of the Background
Electrophotographic developer is typically used for image forming methods such as electrophotography, electrostatic recording and electrostatic printing. The image forming methods typically include the following processes:    (1) an electrostatic latent image formed on an image bearing member such as photoreceptors or dielectric materials is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (2) the toner image is transferred on a receiving material such as receiving papers optionally via an intermediate transfer medium (transfer process); and    (3) the toner image is fixed on the receiving material upon application of heat and/or pressure, or the like (fixing process).
Dry developers are broadly classified into two-component developers which typically consist of a dry toner and a carrier, and one-component developers which are magnetic or non-magnetic and which are typically constituted of a toner and do not include a carrier.
Conventional electrophotographic dry toners for use in electrophotography, electrostatic recording and electrostatic printing are typically prepared by the following pulverization method:    (1) a toner constituent mixture including a colorant, a binder resin (e.g., styrene resins and polyester resins) and optional additive is kneaded upon application of heat thereto (kneading process); and    (2) after being cooled, the kneaded mixture is pulverized to prepare toner particles.
Recently, it is attempted to decrease the particle diameter of toner in order to produce high quality toner images. The toner particles prepared by the pulverization method mentioned above have irregular forms, and therefore the toner particles are further pulverized in image forming apparatus due to the stresses applied to the toner particles by carriers included in developers, developing rollers, toner supplying rollers, toner layer thickness controlling blades and frictional charge applying blades included in the image forming apparatus. As a result, super fine toner particles are produced and/or a fluidity improving agent located on the surface of the toner particles is embedded into the toner particles, resulting in deterioration of image qualities. In addition, such pulverized toners have poor fluidity due to their particle form, and therefore it is necessary to include a large amount of fluidity improving agent therein. Further, the toners have low packing ability (i.e., the amount of a toner contained in a container is relatively small), and thereby the toner bottle has to be enlarged in size. Therefore, it becomes difficult to design a compact image forming apparatus.
Namely, the advantage of the toner having a small particle diameter is not effectively exploited. Further, there is a limit to the particle diameter of a toner prepared by a pulverization method (namely, the particle diameter of a toner cannot be further decreased by a pulverization method).
Recently, color images are popularly produced in offices. Color image forming apparatus have a complex structure and use a complex image transfer device because plural toner images have to be transferred on proper positions of a receiving material. When a toner prepared by a pulverization method is used for such color image forming apparatus, a problem such that the transferred toner images have omissions due to poor transferability of the toner used occurs. In attempting to avoid this problem by increasing the amount of toner adhered to the electrostatic latent images, another problem in that the toner consumption increases occurs.
Therefore a need exists for enhancement of toner image transfer efficiency, which results in production of high quality images and reduction of toner consumption (i.e., reduction of running costs). When a toner having an excellent transfer efficiency is used, it becomes unnecessary to use a cleaning device, and thereby the image forming apparatus can be miniaturized and the manufacturing costs of the apparatus can be reduced. In addition, the image forming apparatus have such an advantage as to produce no waste toner.
In attempting to solve the problems specific to the toners having a small particle diameter and irregular forms, various toners and various toner preparing methods have been proposed.
For example, suspension polymerization methods and emulsion polymerization/aggregation methods in which particles are prepared by emulsion polymerization, followed by aggregation of the emulsified particles have been investigated. In addition, polymer solution emulsifying techniques utilizing reduction of volume of toner particles have been proposed. Specifically, the methods include the following steps:    (1) toner constituents are dissolved or dispersed in a volatile solvent such as organic solvents having a low boiling point;    (2) the solution or dispersion is dispersed in an aqueous medium including a dispersant to form an emulsion; and    (3) the volatile solvent is removed from the emulsion to prepare a dispersion including toner particles.
One of the polymer solution emulsifying methods is disclosed in published unexamined Japanese Patent Application No. (hereinafter JP-A) 07-152202.
The method has the following advantages over the suspension polymerization methods and emulsion polymerization/aggregation methods:    (1) a variety of resins can be used as the binder resin of the toner; and    (2) particularly, polyester resins which are suitable for toners for use in full color image forming because the resins have good transparency and the resultant toner images have smooth surface can be used as the binder resin.
However, the method has a drawback in that the resultant toner has a substantially spherical form, and therefore the toner has poor cleanability when cleaning is performed using a cleaning blade. In addition, the fluidity improving agent which is present on a surface of toner particles is easily embedded into the toner particles, resulting in deterioration of fluidity, and thereby the replenishing property, developing property and charging property of the toner are also deteriorated.
A modified polymer solution emulsifying method is disclosed in JP-A 11-149179 in which a low molecular weight resin is used to reduce the viscosity of the polymer solution or dispersion and to easily perform the emulsification, and the low molecular weight resin is then polymerized in the particles of the emulsion to improve the fixability of the resultant toner. By using this method, the polymerization reaction tends to proceed at the surface of the particles, and thereby the resultant particles have a hard surface. Therefore, the problem in that the fluidity improving agent is embedded into the toner particles can be avoided. However, there is a large amount of free particles of the fluidity improving agent in the toner, thereby causing a problem in that the free fluidity improving agent particles adhere to various image forming members such as photoreceptors and developing rollers, resulting in deterioration of image qualities.
The toners mentioned above are prepared by granulated in an aqueous medium. However, the toners prepared by granulated in an aqueous medium have a drawback in that the charge properties thereof cannot be controlled. Specifically, toners prepared by conventional pulverizing methods which includes the steps of melt-kneading toner constituents including a charge controlling agent to uniformly disperse the charge controlling agent therein; and pulverizing the kneaded mixture such that the charge controlling agent is present on the surface of the resultant toner particles with a certain probability. In contrast, the toners prepared by the in-water granulation methods tend to include a charge controlling agent inside the toner particles (i.e., the charge controlling agent is hardly present on the surface of the toner particles) if the charge controlling agent has a high hydrophobic property. Therefore, good charge property cannot be imparted to the toner particles.
To the contrary, when the charge controlling agent has a hydrophilic property, the charge controlling agent tends to migrate into the aqueous phase during the granulation process, and thereby the resultant toner particles hardly include the charge controlling agent. Namely, it is hard to include a charge controlling agent in a surface portion of toner particles by the in-water granulation methods.
Recently, a strong need exists for an energy-saving electrophotographic image forming apparatus (such as copiers and printers). Therefore, a need exists for a toner having further improved low temperature fixability. In order to improve the low temperature fixability of a toner, it is necessary to decrease the melt viscosity of the toner. In this case, an offset problem occurs in that a toner image is undesirably transferred to a fixing roller and the image is re-transferred to a portion of other images, resulting in formation of undesired images. It is effective to lower the glass transition temperature (Tg) of a binder resin included in a toner, in order to improve the low temperature fixability of the toner, but the preservability of the resultant toner deteriorates.
In order to impart good charge property to a toner, techniques in which a charge controlling agent is externally added to toner particles have been proposed. In addition, in order to prevent deterioration of high temperature preservability caused when it is tried to improve low temperature fixability, methods in which a layer having a relatively high heat resistance property is formed on a surface of toner particles have been investigated.
Japanese patent No. 3104883 (i.e., JP-A 05-107808) discloses a toner in which resin particles having a surface treated with a fluorine-containing surfactant are fixed on the toner particles. However, in this case the resin particles tend to be unevenly present on the surface of the toner particles.
JP-A 06-242632 discloses a toner in which a complex particulate resin prepared by reacting a particulate resin having an acid group with a fluorine-containing quaternary ammonium salt is fixed on the surface of the toner particles in the presence of a nonionic surfactant. However, this technique is used for controlling the charge property of the toner, and therefore there is no description about influence of a particulate inorganic material, which is added to the toner particles as an external additive to improve the fluidity of the toner, on the complex particulate resin. In addition, the resin particles tend to be unevenly present on the surface of the toner particles.
Further, JP-A 2003-84502 discloses a toner in which a particulate material having a charge with a first polarity opposite to that of the mother toner particles is adhered to mother toner particles to impart a charge with the first polarity to the resultant toner. However, the resultant toner has uneven charge property, namely, there are many toner particles having a charge with a polarity opposite to the desired polarity.
Because of these reasons, a need exists for a toner which has good high temperature preservability and which has so good charge property, transfer property and fixing property as to produce high quality (color) images in a relatively small amount of heat energy.